It is known to replace all or part of a knee joint in which the joint surfaces have deteriorated, for example as a result of osteoarthritis. Such deterioration usually starts in only one of the tibeo-femoral compartments and may spread to the other at a later stage. Replacement of only one compartment of the joint can therefore be sufficient to provide prolonged relief from symptoms. Damaged bearing surfaces are replaced by a unicompartmental prosthesis which comprises a femoral implant and a tibial implant (usually metallic), which interface through a (polyethylene) bearing component disposed between the two implants.
A unicompartmental or partial knee replacement (PKR) helps to conserve undamaged bone and restores more natural movement to the joint. Also, owing to the small size of the prosthesis, the surgery may be less invasive than a total knee replacement (TKR). However, the design requirements for partial knee replacement prostheses are more demanding than those for total knee replacement prostheses. Unlike in a total knee replacement, where one or more ligaments can be discarded and the mechanics of the knee can be simplified, in a unicompartmental knee replacement, all the ligaments in the joint must be retained and restored to their natural tensions and the bearing component must be completely unconstrained.
During articulation of the knee, and particularly when the joint is at full extension, the bearing component can impinge on femoral condylar bone tissue superior to the femoral implant, as illustrated in FIG. 1. Such impingement of the polyethylene bearing component onto the bone can lead to post operative pain, damage to the bearing, increased wear and eventual failure. It is therefore essential to remove a sufficient amount of anterior bone on the femoral condyle during the implantation procedure to prevent such impingement from occurring.
Orthopaedic surgeons conventionally use a bone chisel to manually remove the anterior bone. However, such a manual procedure can easily be forgotten during surgery and, even when carefully completed, results in an undesirable non uniform bone edge and in the removal of an uncertain and varying amount of bone.